In many industrial operations it is necessary to remove large amounts of heat and circulating water is commonly used as a heat transfer medium. Various chemical treating agents are added to the water used for this purpose including chromium containing corrosion inhibitors in the form of chromate salts. In operation, the chemically treated process water is circulated through heat exchangers at the heat source and then passed to a cooling tower where it is cooled and recycled.
Since evaporation occurs in the cooling tower, there is a resulting increase in concentration of harmful scaling compounds and other dissolved solids present in the process water due to the losses in water volume. In order to maintain the solids concentration at acceptable levels, a portion of the process water is continuously withdrawn from the cooling tower and discharged as waste. This flow of waste water is generally referred to as the "blow-down" stream. It is, of course, necessary that fresh make-up water be added to the system to compensate for the volume lost by evaporation and blow-down. Also, chromate salts and other treating agents must be added to the make-up to compensate for the losses incurred in the blow-down flow.
The need to replace the relatively expensive corrosion inhibitors lost in the blow-down obviously adds to the cost of the cooling operation. In addition, the relatively high concentration of these materials in the blow-down stream causes difficult disposal problems. The reason for this is that hexavalent chromium is known to be toxic and its discharge into sewers and waterways has come under strict environmental control.